I. Field of the Invention
The present invention relates to a phase lock loop. More particularly, the present invention relates to a novel and improved digital phase lock loop (PLL) which locks a numerically controlled oscillator (NCO) to a non-continuous reference signal.
II. Description of the Related Art
The emergence of digital video has brought many benefits to the consumers. Digital video has many desirable characteristics such as multiple programs and improved picture quality. The use of complex digital compression techniques allows multiple programs (up to six or ten programs depending on the desired video quality) to be transmitted on a single 6.2 MHz channel. Digital transmission allows the reception of the video signal at a lower signal-to-noise ratio than with analog transmission. The lower threshold allows the use of a smaller satellite dish to receive the digital transmission over a satellite link. In addition, digital video allows easy incorporation of graphical features and text in the video signal because the digital processing capability is present in the digital video system. In the interim period, until digital video completely displaces analog video, a vast number of existing video services provide analog video. A digital video system which desires to incorporate existing analog programs among its programming features needs to receive analog video as well as digital video.
For digital video, the data representing the video picture is transformed into a signal which the television accepts. In a digital video system, this function is performed by a video encoder. The video encoder receives the data for the luminance and chrominance components, modulates the chrominance components by a color subcarrier to obtain a chrominance signal, adds the chrominance signal to the luminance component to obtain a digital signal, filters the digital signal, and converts the filtered signal into an analog video signal. In the U.S., the analog video signal conforms to the National Television Standard Committee (NTSC) format. In Europe and South America, the analog video signal conforms to the Phase Alternating Line (PAL) format.
For NTSC signal, the frequency of the color subcarrier is specified to be 3.579545 MHz.+-.5 Hz. For PAL signal, the frequency of the color subcarrier is different for different versions of PAL. For example, for PAL-B, D, G, H, and I which are used in Europe, the color subcarrier is 4.433618 MHz. The color subcarrier is digitally generated within the video encoder by a direct digital synthesizer comprising a numerically controlled oscillator (NCO) and a sine look-up table which is commonly referred to as a color look-up table (CLUT). For each system clock cycle, the NCO accumulator is updated and the content of the NCO accumulator is used to address the CLUT. The NCO accumulator contains the phase of the color subcarrier and the CLUT transforms the phase information into the corresponding sinusoidal amplitude of the color subcarrier.
Graphical user interface (GUI) is an important feature of a digital video system. The GUI allows the consumers to perform complicated programming tasks on the digital video system through simple to understand commands which are graphically displayed on the television screen. The GUI can comprise graphics, text, or a combination of graphics and text arranged in an easy to understand and use display. The graphics and text are typically generated by a microcomputer or a GUI engine. The generated graphics and/or text are then stored in memory and sent to the video encoder at the appropriate time. The video encoder encodes the digital representation of the graphics and text into a corresponding analog signal which the television accepts.
In a digital video system, the digital data representing the video is quantized at a predetermined rate. Subsequent signal processing on the digital data is performed at a rate which is typically a multiple of the quantization rate. For Motion Picture Encoding Group (MPEG) standard, the video is quantized at 13.5 MHz and many signal processing steps are performed at 27 MHz. To simplify the GUI generation, the GUI is typically generated at the same 13.5 MHz data rate as the video data. The graphics and text for the GUI are multiplexed with the video data and sent to the video encoder.
A challenge arises when a digital video system is designed to support both digital video and analog video. When operating in the analog mode, the digital video system receives the frequency modulated (FM) signal, demodulates the FM signal to obtain the analog NTSC or PAL video signal, filters and amplifies the video signal, and provides the video signal to the television. Since the digital video system has the signal processing capabilities to generate GUI in the digital mode, it is desirable to have the same GUI be available during the analog mode. The use of the same GUI interface in both analog and digital video modes makes the video system more user friendly. A user does not have to learn different programming commands depending on whether the user is watching video from an analog source or a digital source.
In one approach used in the prior art to incorporate GUI on the analog video signal, a 14.318 MHz oscillator is locked to the color bursts of the analog video signal. For an NTSC signal, color bursts appear for nine cycles per active video line (each line is 227.5 cycles wide). In this specification, an active video line is a video line which contains color bursts. The 14.318 MHz oscillator is precisely four times the color burst frequency and can easily be phase locked to the color burst using a gated phase lock loop. The gated phase lock loop is only enabled for the duration when the color bursts are present. A continuous 14.318 MHz clock is then generated by buffering the output of the 14.318 MHz oscillator. The continuous 14.318 MHz clock is provided to the video encoder. The video encoder digitally locks the NCO to the continuous 14.318 MHz clock through a second phase lock loop.
This technique for locking the NCO within the video encoder to the color bursts of the analog video signal requires two phase lock loops. The dual loops approach requires a complex hardware design which results in higher cost. The dual loops approach is inherently more unreliable than a single loop approach. Furthermore, the dual loops approach has a longer acquisition time which is defined as the time required to lock the NCO to the analog video signal. The present invention addresses these shortcomings by using a single phase lock loop design to lock the NCO to a non-continuous reference signal.